This invention relates to remote control arrangements and more particularly to an arrangement to control a function at a remote location from a local location.
Devices are presently employed to control a function at a remote location from a local location employing as the most convenient means of communication between the control location and the function location a transmission line, such as a leased telephone channel or line. Thus, the equipment employed acts as an interface between control and function and the leased telephone line. Audio band tone operation is generally employed because of the bandwidth characteristics of carrier and microwave long distance telephone channels. This is usually accomplished by employing audio frequency tone oscillators at one end of the system and narrow band tone filters at the opposite end of the system that are tunned to the same frequency as the oscillators. Also employed are switches gating the tone signals on and off of the leased telephone channel. The oscillators and filters are generally isolated and impedance matched via transformers. In systems where voice signals are present (remote paging and communication), there is the added requirement that the receiving tone filters be immune to the voice signal so as not to cause false operation or control of the remote function.
A disadvantage of these prior art arrangements is the lack of instantaneous operation. This is due to the requirement that a legitimate single frequency signal or tone must have a certain minimum time duration (approximately 0.8 seconds) in order to prevent false operation of the receiving tone filters by voice signals. Other disadvantages are the physical bulk and the high cost of these prior art devices which usually employ discrete circuit elements.